The present invention relates to a lead-free nickel alloy produced chemically, i.e. by an externally electroless method, a process for the production of such a nickel alloy by electroless metal deposition in an aqueous electrolyte and articles plated therewith.
The externally electroless chemical nickel plating of metal surfaces is a process for the protection of metals against corrosion which is frequently used on an industrial scale.
To achieve an improved protection against corrosion, it has proved necessary to achieve a nickel/phosphorus alloy by adding suitable compounds to the aqueous electrolyte.
Pure nickel and phosphinate-containing electrolytes for chemical nickel plating require additional stabilisers to prevent spontaneous decomposition. Stabilisation sufficient for industrial application has previously been achieved only by adding lead compounds. Stabilisers previously added as an alternative, such as molybdenum, cadmium or tin compounds, exhibit an unsatisfactory effectiveness in comparison with lead.
However, even such a modified chemical nickel layer does not satisfy the increased requirements regarding resistance to corrosion such as they exist in the electronics industry for the manufacture of printed circuits, for example.
The addition of lead, however, has the disadvantage that edge passivity can be increasingly observed when the value specified for the lead concentration is exceeded. Edge passivity means that a reduced layer structure can be observed at the edges of structural parts. This results in a reduced resistance to corrosion in the areas concerned.
This problem can be observed in particular on the inner edge areas of bores in printed circuits as a result of which the electrical properties of soldered and/or bonding joints are permanently negatively affected.
Moreover, the addition of lead is no longer tenable due to environmental considerations; the resulting presence of lead in printed circuits prevents an environmentally appropriate disposal of the corresponding printed circuit boards.
An electrolyte for the production of lead-free nickel layers produced by an externally electroless method is known from U.S. Pat. No. 2,884,344. According to the process described therein, at least two cations are added to the electrolyte which are selected from the group of antimony, arsenic and bismuth. The concentration of antimony and bismuth ions is at least 5 ppm, the ratio between antimony and bismuth being between 1:5 and 1:0.5.
The layers produced according to this process exhibit a proportion of bismuth in the nickel alloy of at least 0.5% by weight and a proportion of antimony of maximum of 0.1% by weight of all constituents of the nickel alloy, have a matt surface, exhibit a distinct inherent tensile stress and have an unsatisfactory resistance to corrosion.